The present invention relates to an apparatus for correcting a variation in the time base of a reproduced signal, which apparatus is suited for use in a system for reproducing recorded data or an information signal from a recording medium. In particular, the invention concerns an apparatus for correcting a variation in the time base of a reproduced color video signal, which apparatus is suited for use in a system for reproducing a color video signal from a recording medium such as a video disc.
As a typical example of a system for reproducing a color video signal from a recording medium on which the color video signal is recorded, there is known a video disc play-back apparatus in which a rotatable disc-like recording medium generally is used. Information signals or color video signal is recorded on the disc along a spiral track after having been modulated in an appropriate manner and is read out by means of a detecting head.
Usually, a carrier signal is modulated by the information or data signal and the recording on the video disc is made in the form of a series of concave and convex recesses termed "pits". For reproduction, there are known a system for reading out the recorded information as changes in an electrostatic capacitance by means of a stylus and a system in which the video disc is irradiated by a laser ray and the recorded information is read out as changes in the intensity of reflected or transmitted light. The video disc is rotated by a drive motor whose speed is controlled so that the disc is rotated in a stable manner. However, because of the inevitable tolerance or dimensional error of the center hole of the disc as well as a discrepancy between the center of disc and the center of rotation at the coupling between the disc and the rotating means, the video disc often is rotated necessarily with a slight eccentricity. The rotation of the disc in the eccentric state as well as variation in the speed of rotation of the drive motor which is inevitable even when the drive motor is controlled so as to be rotated in a stable manner and deformation of the spiral track will give rise to variation in the time base of the reproduced.
Such variation in the time base of the reproduced signal has heretofore been corrected by detecting the variation and correspondingly displacing the detector head for reading out the information relative to the disc. Additionally, in conjunction with the frequency conversion which is usually effected for processing the picked-up signal, correction or compensation of the variation in the time base is performed by using an automatic phase control or APC loop.
In these controls described above, a signal representative of variation in the time base is derived from a color burst signal and a horizontal synchronizing signal contained in the reproduced signal. Consequently, the control is a so-called sample control performed at a horizontal frequency of about 16 KHz. Besides, since the APC loop includes a voltage controlled oscillator or VCO, difficulty is encountered in increasing sufficiently the open loop gain of the APC loop for a frequency higher than about 500 Hz. This can be explained by the fact that the loop including the VCO necessarily contains an integrating factor (l/s) which provides a cause for an attenuation of the signal amplitude (-6 dB/octave).
Under the circumstances, when a variation in the time base occurs beyond an allowable level in a frequency range higher than about 500 Hz for various reasons inclusive of distortion involved in the recording on the disc, it becomes very difficult to correct or compensate such time base variation. As a consequence, a reproduced picture suffers color phase irregularity to a serious disadvantage.